Does Leaf Chemistry Differentially Affect Breakdown in Tropical Vs Temperate Streams? Importance of Standardized Analytical Techniques to Measure Leaf Chemistry
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J. N. Am. Benthol. Soc., 2009, 28(2):440–453 Ó 2009 by The North American Benthological Society DOI: 10.1899/07-083.1 Published online: 7 April 2009 Does leaf chemistry differentially affect breakdown in tropical vs temperate streams? Importance of standardized analytical techniques to measure leaf chemistry 1 2 Marcelo Ardo´n AND Catherine M. Pringle Odum School of Ecology, University of Georgia, Athens, Georgia 30602 USA Susan L. Eggert3 Department of Entomology, University of Georgia, Athens, Georgia 30602 USA Abstract. Comparisons of the effects of leaf litter chemistry on leaf breakdown rates in tropical vs temperate streams are hindered by incompatibility among studies and across sites of analytical methods used to measure leaf chemistry. We used standardized analytical techniques to measure chemistry and breakdown rate of leaves from common riparian tree species at 2 sites, 1 tropical and 1 temperate, where a relatively large amount of information is available on litter chemistry and breakdown rates in streams (La Selva Biological Station, Costa Rica, and Coweeta Hydrologic Laboratory, North Carolina, USA). We selected 8 common riparian tree species from La Selva and 7 from Coweeta that spanned the range of chemistries of leaf litter naturally entering streams at each site. We predicted that concentrations of secondary compounds would be higher in the tropical species than in the temperate species and that high concentrations of condensed tannins would decrease breakdown rates in both sites. Contrary to our predictions, mean concentration of condensed tannins was significantly greater (2.63, p , 0.001) for species at Coweeta than for species at La Selva. Concentration of condensed tannins was negatively correlated with breakdown rate among Coweeta species (r ¼0.80), not among La Selva species, and negatively correlated when the 2 sites were combined (r ¼0.53). Concentrations of structural compounds were strongly correlated with breakdown rate at both sites (Coweeta species, lignin r ¼0.94, cellulose r ¼0.77; La Selva species, cellulose r ¼0.78, C r ¼0.73). The chemistries of 8 riparian species from La Selva and 7 riparian species from Coweeta were not as different as expected. Our results underline the importance of standardized analytical techniques when making cross-site comparisons of leaf chemistry. Key words: organic matter, decomposition, tannins, phenolics, lignin, nitrogen, carbon. Leaf litter is an important energy source for 2) low N content (Melillo et al. 1983), 3) high lignin:N organisms in forested streams (Wallace et al. 1999). and C:N ratios (Enriquez et al. 1993), and 4) high The chemistry of leaf litter affects its decomposition concentrations of tannins (Ostrofsky 1997, Driebe and Whitham 2000). The effect of leaf litter chemistry on rate, persistence, and availability to consumers (Web- breakdown has been studied extensively in temperate ster and Benfield 1986). Leaf chemical characteristics streams, but leaf litter in tropical streams has not known to decrease breakdown rate include: 1) high received as much attention. The physical and biolog- concentrations of lignin (Gessner and Chauvet 1994), ical differences between tropical and temperate 1 streams (Covich 1988) provide good reason to expect Present address: Duke University, Department of that the factors controlling leaf litter breakdown might Biology, Durham, North Carolina 27708 USA. E-mail: differ between these 2 regions. [email protected] 2 E-mail address: [email protected] Leaves of many tropical plant species tend to have 3 Present address: Northern Research Station, Forest higher concentrations of secondary compounds and Service, US Department of Agriculture,1831 Hwy 169 E, more recalcitrant compounds than do leaves of Grand Rapids, Minnesota 55744 USA. E-mail: seggert@fs. temperate species (Levin 1976, Coley and Aide 1991). fed.us Previous researchers hypothesized that high concen- 440 2009] TROPICAL–TEMPERATE COMPARISON OF LEAF CHEMISTRY 441 trations of secondary compounds (e.g., tannins) in Methods many tropical species inhibit leaf breakdown rates in Site descriptions tropical streams (Stout 1989, Irons et al. 1994). Stout (1989), using primarily published data, hypothesized La Selva Biological Station (lat 108260N, long that high concentrations of condensed tannins in 848010W; 1536 ha) is in the Caribbean lowlands of tropical species decrease leaf breakdown rate in Costa Rica, at the lowland end of the last protected tropical streams. Most studies that have examined this unbroken biological corridor spanning altitudinal hypothesis have incubated tropical leaves in temperate extremes in Central America. La Selva receives 4000 streams (but see Wantzen and Wagner 2006). In a mm of rain annually, with .400 mm/mo from May to temperate Australian stream, condensed tannin con- December (Sanford et al. 1994). Mean annual stream tent was negatively correlated with breakdown rates water temperature ranges from 24 to 268C. Streams at of 6 species of Eucalyptus, but not with breakdown La Selva display a wide range of variation in solute rates of 12 other tropical and temperate species concentrations and pH because of geothermally (Campbell and Fuchshuber 1995). Leaf litter from modified groundwater inputs (Pringle and Triska tropical species with higher concentrations of tannins 1991). Algal assemblages in streams at La Selva are was processed more slowly by shredders in a German light limited by dense canopy cover (.90%). Thus, stream than was leaf litter from temperate species with food webs are detritus based (Pringle et al. 1993). Most lower concentrations of tannins (Wantzen et al. 2002). trees in La Selva are nondeciduous, and leaf fall is Results from these previous studies might not be an continuous throughout the year (Hartshorn 1983). 0 accurate reflection of differences in leaf litter chemistry Coweeta Hydrologic Laboratory (lat 35800 N, long 0 and leaf breakdown dynamics between tropical and 83830 W; 2185 ha watershed) is a US Department of temperate streams for 3 reasons: 1) literature reviews Agriculture Forest Service research facility and Long are hindered by the use of different analytical Term Ecological Research site in the Blue Ridge techniques to measure secondary compounds across Province of the Southern Appalachian Mountains studies (Stout 1989), 2) chemical changes in leaves (North Carolina). Coweeta Hydrologic Laboratory during decomposition occur faster in tropical than in has 50þ y of hydrologic and climate monitoring data. temperate streams (Ardo´n and Pringle 2008), and 3) Coweeta receives ;1700 mm of rain annually, and the microbial and invertebrate fauna of temperate stream temperatures range from 4 to 178C (Swank and streams (i.e., where tropical leaves were incubated) Crossley 1988). Dissolved concentrations of N and P might not have been adapted to the chemical are usually low (,10 lg/L), and pH averages ;6.6 to characteristics of tropical leaves. The effects of second- 6.8 (Wallace et al. 1986). Streams in Coweeta drain ary compounds (measured with standard analytical mixed-hardwood forests dominated by oaks and techniques) on breakdown rate of native species hickories and are heavily shaded by riparian rhodo- decomposing in their natural habitats at both a tropical dendron (Wallace et al. 1982). Thus, food webs are and temperate site remain to be explored. detritus based (Wallace et al. 1999). The objectives of our study were to: 1) directly compare initial leaf litter chemistry of common native Selection of tree species for chemical analyses and riparian tree species spanning a wide range of quality breakdown experiments from a tropical site (La Selva Biological Station, Costa At least 2015 native plant species exist at La Selva, Rica) and a temperate site (Coweeta Hydrologic but the exact number that can be considered riparian is Laboratory, North Carolina, USA) using standardized unclear (C. Taylor, Missouri Botanical Garden, person- analytical techniques and 2) examine the importance of al communication). Up to 116 plant species have been leaf secondary compounds on leaf breakdown at these reported near streams and wetlands in vegetation 2 sites, where a relatively large amount of information studies at La Selva (Lieberman et al. 1985). A list of exists on plant chemistry and breakdown dynamics in trees and shrubs species that commonly occur along streams (Hartshorn 1983, Swank and Crossley 1988, streams in La Selva was compiled based on the Rosemond et al. 1998, Webster et al. 1999). We literature (Hartshorn 1983, Lieberman et al. 1985, predicted that concentrations of secondary compounds Hartshorn and Hammel 1994) and consultation with (total phenolics, condensed and hydrolyzable tannins) experts on the La Selva flora (B. Hammel and M. would be higher among species from the tropical site Grayum, Missouri Botanical Garden; O. Vargas and N. than among species from the temperate site, and that Zamora, Organization for Tropical Studies and Costa secondary compounds would inhibit breakdown rates Rica’s Instituto Nacional de Biodiversidad; Appendix). in both sites. Species selected from this list for inclusion in the litter 442 M. ARDO´ NETAL. [Volume 28 breakdown experiment were chosen to span the range tannins and hydrolyzable tannins can be higher than of litter chemical quality of leaves that naturally enter total phenolics in our assays because they are La Selva streams. These species had high (Zygia measured with different reactions (Schultz and Bald- longifolia [Fabaceae, sotacaballo], Carapa nicaraguensis